With respect to copying machines, for example, it has been proposed to employ image reading equipment with a reducing optical system therein. The image reading equipment with this reducing optical system includes a fluorescent lamp or a halogen lamp, which irradiates the original sheet in order to form optical images for reading, a mirror unit, which is composed of a full rate mirror (FRM) and a half rate mirror (HRM) and transmits the optical images, an optical lens unit, which reduces the optical images received from this mirror unit and projects them to form their images on the organic photo-sensitive material belt, and so forth.
On the other hand, digital color copying machines have been the subject of research in recent years, and certain types have been developed. The present applicant has filed application for a patent on a digital copying machine (for example, U.S. patent application Ser. No. 292195-1988). This digital color copying machine is provided with an image input terminal (IIT), which reads the images on the color original sheet for each of the three primary colors of light, namely, red (R), green (G), and blue (B), and then converts the read data into electric digital density signals through photoelectric conversion.
This IIT is comprised of an imaging unit, which is provided with an exposing lamp, a rod lens array and a CCD line sensor which scans the original sheet and thereby optically reads the images thereon, a driving means, which drives the imaging unit for its movement, electrical hardware, which performs the process of conversion of the electric read signals generated by the CCD line sensor, and so forth.
With such a digital color copying machine, it is possible to perform various processes in an easy and simple way since the machine applies digital processing to the image signals, and it is consequently possible to obtain color copies with high image quality.
The imaging unit in the IIT employed in this digital color copying machine is a close-coupled type image reader, and its construction is therefore relatively large. However, it is desired to build the equipment in as compact a size as possible. Moreover, with respect to those copying machines which are capable of processing original sheets up to the Japanese standard A3 paper size, i.e. those forms of paper in sizes in most frequent general use, it is necessary to provide the copying machine with a CCD line sensor with a length not shorter than the shorter side, i.e., 297 mm, of the A3 paper. Accordingly, this means that not only is the CCD line sensor itself more costly, but also processing operations using it are much more expensive. Therefore, it is desirable to apply reducing type image reading equipment as described above to the IIT of the digital color copying machine just described.
A three-line sensor (a three-color linear sequential sensor), which is provided with lines of picture elements arranged above and below with color filters having spectral sensitivity for R, G, and B has been developed for use as a reading sensor composed of CCD line sensors. For the application of this sensor to a color copying machine, the depth of focus is set for the lens in such a manner that the resolution (MTF) of the lens unit will be 40 per cent or more for the original sheet in 51 p/mm, for example.
However, in image reading equipment in which a reducing type optical system is employed, the equipment sometimes fails to perform an accurate reading of images. This happens because it is not possible to attain the required resolution (MTF) unless the reading sensor is installed precisely in its specified position. Above all, as shown in FIG. 24 of this application, considerable differences may occur in the location of the sensors for the individual colors in the construction of the three-line sensor 217, which has three lines of picture elements for R, G, and B, respectively. These lines of picture elements are arranged in the vertical direction, with a fairly large distance between the line sensors R and B, which are respectively positioned on the two ends. The depth of focus will be different by [d] between R and B if the optical images read from the original sheet are input slanted in the upward and downward direction, as illustrated with the two-dot chain line in FIG. 24.